Myelodysplastic syndromes (MDS) comprise a heterogeneous group of clonal hematological disorders that is diagnosed cytomorphologically. They are characterized by ineffective hematopoiesis resulting in peripheral blood cytopenias and showing by definition in all cases dysplastic features in at least one lineage in the bone marrow, an increased myeloid blast count in the bone marrow or typical chromosomal aberrations. They all share a variable risk for transformation into an acute myeloid leukemia.